This invention relates to power generation plants, and, more particularly, to a system and method of operating power generation plants so as to be resistant to thermoacoustic oscillations over the entire load range of such plants.
Combined cycle power generation systems are well known in the art and typically involve the combustion of natural gas or oil under pressure for the generation of hot gases which are passed through a gas turbine where the hot gases expand and cool while performing work in the generation of electrical power. The turbine exhaust gases are passed to a heat exchanger for the generation of high temperature steam which is used by steam turbines to perform additional work.
Often, severe thermoacoustic oscillations are encountered during the cold start-up of the gas turbine in which large pressure pulsations occur between the turbine discharge duct and heat recovery area. These oscillations result in severe structural vibration of the ducting from the turbine to the heat exchanger and surrounding structures, including the turbine. Such thermoacoustic oscillations typically result in the shut-down of the turbine. After numerous starts and subsequent shut-downs of the turbine, the vibrations disappear and the turbine operates without further thermoacoustic oscillations.
Similarly, utility steam generators are well known. In this type of arrangement, one or more burners are usually disposed in communication with the interior of a furnace and operate to combust oil or fuel gas in the presence of air to produce heat that is utilized to convert water to steam. The combustion air for the burners is typically discharged from a plenum towards a combustion zone, which is usually located immediately adjacent to the furnace wall. Perturbations in the stability of the combustion zone often result in the excitation of a strong acoustical oscillation in the furnace.
Severe thermoacoustic oscillations are typically characterized by the presence of well developed acoustical standing waves in the furnace, generating high sound levels and causing structural vibration of the furnace walls. Because of noise considerations, as well as concern for the structural integrity of the furnace, it is undesirable for severe thermoacoustical oscillations to take place within the operating range of the steam generator.
In general, systems which include hot and cold interconnected enclosures that are filled with gas may be subject to significant thermoacoustic oscillations. The driving force for such instabilities is the thermal energy fed or withdrawn from the system. The phenomenon of acoustic vibration or pressure oscillations in a gas caused by a constant heat source is characterized in two ways--each named after its discoverer--Sondhauss oscillations and Rijke oscillations. The Sondhauss pressure oscillation occurs when heat is added (either externally or internally) to a gas-filled tube having at least one end closed and no net flow of gas through the tube (Sondhauss, 1850). The Rijke oscillation occurs when heat is added to an internal grid located in the lower half of a vertical tube having both ends open, with flow of gas upward through the tube (Rijke, 1859).
Thermoacoustic oscillations are known to occur in various industrial installations, but they are generally difficult to diagnose. They occur either in their pure Sondhauss or Rijke form or also in a modified form such as described in the present invention. Thermoacoustic oscillations are occasionally considered a desired phenomenon in such applications as pulsed combustion. However, in a vast majority of applications, particularly in combustion processes, severe thermoacoustic oscillations are considered an unwanted phenomenon because of the generation of loud noise, or the induction of structural fatigue and damage, or both.